1. Field of the Invention
The present invention concerns hydraulic assistance devices and more particularly such a device for a braking circuit with mixed hydrodynamic (more commonly known by the description "full power") and hydrostatic operation.
2. Description of the Prior Art
A hydraulic assistance device of the "full power" type is known from French Patent Publication No. 2,570,663 of Gilbert Kervagoret, the device being capable of operating as a conventional master cylinder, in the hydrostatic mode, in the case of failure of the high pressure circuit. The device incorporates an annular cup which separates a first master cylinder pressure chamber from a second pressure chamber which is connected to the brakes of the vehicle and is capable of being connected selectively to the high pressure source. In the case of failure of the high pressure circuit, when the device is operated, a piston which defines the first chamber moves and compresses the fluid in the first chamber, which causes tilting of the cup and allows the fluid under pressure to pass in the hydrostatic mode towards the brakes.
Despite its obvious advantages, this system nevertheless has slight disadvantages in that the thresholds of force may become too high and that the distribution leakage may also be considered as excessive.